Traveling-wave semiconductor laser amplifiers (TWSLAs) will find applications as optical preamplifiers in high sensitivity receivers, as signal boosters for interprocessor optical interconnects, in local area networks, and photonic switching systems. Because of their very large bandwidth (.about.10 THz), they might also play an important role in future high capacity wavelength division multiplexed systems. The superluminescent LED is the key element in several applications such as a tunable external cavity semiconductor laser which provides a tunable, stable, and ultra-narrow linewidth(less than 100kHz) laser source, and for optical intensity enhancement using high Q optical cavity. An ultra-low AR coating further provide the features of high gain and high power for both devices since the cavity resonances can be suppressed under high injection current. Both arc typically fabricated by depositing anti-reflection (AR)coatings on both facets (for laser amplifiers)or single facet (for superluminescent LED) of a laser so as to suppress the Fabry-Perot transmission peaks originating from the laser cavity. Obtaining ultra-low facet reflectivity is of utmost importance in all of these promising applications. For example, in order to make a 25 dB gain with less than 1 dB spectral gain ripple for a laser amplifier, a facet reflectivity of 10.sup.-4 or less is required. Similar requirement is needed for the applications which utilize superluminescent LEDs as an element, such as the tunable external cavity laser where continuous spectral tuning and low spectral ripple are needed. This implies that the reactive index and the thickness of the AR coating have to be controlled to within .+-.0.02 and .+-.20 .ANG., respectively. Therefore, the controlled deposition of an appropriate coating material with a precise refractive index and thickness is required for obtaining high performance traveling-wave laser amplifiers and superluminescent LEDs.
The refractive index control can be achieved by using SiO, one of the most widely used material for this type of deposition. The adjustments of the refractive index of SiO is usually done by adjusting the oxygen pressure in the deposition chamber. A non-stoichiometric film composition is then obtained which is represented by SiO.sub.x, where x can vary between 1 and 2 as the refractive index is adjusted between 1.9 and 1.45, respectively. In addition to the oxygen pressure, the deposition rate is another important factor which can significantly affect the final value of the refractive index. Because the refractive index depends not only on the oxygen pressure but also on the deposition rate, it has generally been difficult to obtain the same refractive index from run to run lot reproducible low facet reflectivity manufacturing.
Several in situ monitoring techniques have been used for the AR coatings of semiconductor lasers. They include the monitoring of the output light power vs. bias current (see the article entitled "Directly controlled deposition of anti-reflection coatings for semiconductor lasers," by M. Serenyi and H. U. Habermeier, published in Applied Optics, volume 26, 1987 at pages 845-849), the measurement of the facet loss induced forward voltage changes (see the article entitled "In situ reflectivity monitoring of anti-reflection coatings on semiconductor laser facets through facet loss induced forward voltage changes," by J. Landreau and H. Nakajima, Appl. Phys. Lett., published in Applied Optics volume 56, 1990 at pages 2376-2378), and the measurement of the spontaneous emission spectrum. However, none of these techniques monitors in real time the refractive index of the film. Typically, the refractive index is only measured after deposition and its value suffers from variations from run to run. In order to reproducibly obtain a film with a given refractive index, a technique capable of accurately measuring the refractive index of the film during the deposition is needed. The invention herein uses real-time in situ ellipsometry for the refractive index measurement, which permits accurate, fast, and non-destructive measurements of the film characteristics during the deposition. In situ ellipsometry is a technique that has also been widely used in many other applications for real-time monitoring, see the technical review article by R. W. Collins entitled "Automatic rotating element ellipsometers: calibration, operation, and real-time applications," published in Rev. Scient. Instru., volume 61, 1990, pages 2029-2062. In some applications, it is used for process control, such as Yu et al. U.S. Pat. 5,131,752 and Aspnes et al. U.S. Pat. No. 5,091,320. However, most of tile real time applications as taught and suggested require monitoring the ellipsometric data with (.DELTA., .psi.) coordinate, and do not extract the refractive index and thickness of the measured film from .DELTA. and .psi. in real time. This is also the case in Yu's U.S. Pat. No. 752 for endpoint control of a film deposition or etching.
In Aspnos' U.S. Pat. No. '320, a general idea of using the extracted dielectric function and thickness from the .DELTA. and .psi. to control a material growth is taught. However, the invention only demonstrates a method of extracting the dielectric function using an approximation that the dielectric function of the deposited film is sufficiently close to the substrate such that the reflection from the interface between the film and the substrate can be neglected. In practice, for the case of depositing a non-absorptive dielectric film on a semiconductor substrate, the above approximation is not valid. The exact solution has to be solved numerically. Therefore, a substantial computation power is previously considered necessary. The invention herein further demonstrates that for the case of non-absorptive films, the required computation can be implemented on a personal computer with a total measurement time in about half a second which allows for real-time monitoring and deposition control.
In the teaching by I-Fan Wu et al. entitled "Real-time "in situ monitoring of antireflection coatings of semiconductor laser amplifiers by ellipsometry," presented in IEEE Lasers and Electro-Optics Society (LEOS) Annual Meeting, November, 1991, a general approach to the AR coating by using ellipsometry is disclosed. However, this teaching does not provide critical information of the required methodology for the refractive index extraction and control schemes for better reproducibility of AR coating for semiconductor lasers with a lower facet reflectivity in the range of 10.sup.-5 or less.